Refactor backend for delegated measurement methods

[jemappellesami]

Description of the change: Refactoring of the backend to enable delegated measurement-based QC protocols.

1. Decouple backends from patterns, to allow backend instantiation independent of any pattern
2. Better code sharing and less redundancy between backends
3. Introduction of NodeIndex data structure for efficient mapping between nodes and indices in the backend's internal state
4. Simulators are now parametrized by MeasureMethod, with default measurement method that implements MBQC. It allows customized measurement methods, for instance for delegated QC protocols.

See examples of usage of these features for verification of delegated MBQC quantum computations in https://github.com/qat-inria/veriphix .